Conventionally, a urea SCR (Selective Catalytic Reduction) system is known. In the urea SCR system, nitrogen oxides (NOx) contained in exhaust gas are reduced by adding urea water. The urea water is a water solution of urea. When an atmospheric temperature falls down to a specified temperature, the urea water for the urea SCR system is frozen. If the urea water is frozen, the urea water cannot be added to exhaust gas. JP-2005-315206A (EP-1741887A1) shows a urea-water adding device having a heater in order to thaw the frozen urea water.
The heater receives electricity from a battery mounted to a vehicle. When the heater is energized with the engine stopped, the battery may go dead.